1. Field of the Invention
The present invention relates to a polarization control liquid crystal optical switch for use in optical communication and its drive method, and more particularly to a polarization control liquid crystal optical switch for use in a wavelength division multiplexing (WDM) communication method and an optical network using optical fibers.
2. Description of the Prior Art
A polarization control optical modulator using a nematic liquid crystal has been practically used in a liquid crystal display. An example of a polarizing rotator used in a polarization control optical modulator is a 90° twisted nematic (TN) liquid crystal. Although a TN liquid crystal cell is basically suitable for a high contrast ratio and a large cross-talk attenuation, its cell thickness is proportional to the square of the response speed, meaning that a higher response requires a thinner liquid crystal cell. Because the Mauguin condition does not satisfy the condition that λ/2 must be sufficiently small with respect to Δn·d (where, λ is a wavelength, Δn is a refractive index anisotropy of liquid crystal, and d is the thickness of a liquid crystal cell), the wave-guide effect diminishes. As a result, the contras ratio deteriorates. To solve this problem, today's high response TN display, a typical application of the TN liquid crystal, usually creates cells by setting up cell parameters that satisfy the first (hereinafter called 1st) minimum condition which is a term used for the normally black mode of a liquid crystal display and in which the Muaguin parameter u of the expressiond=(λ/2)·(u/Δn)is set to the square root of 3.
An example of applying the TN liquid crystal, which uses the 1st minimum condition, to a 2×2 switch for use in optical fiber communication is described, for example, in “A1.3 μm Single-Mode 2×2 Liquid Crystal Optical Switch”, Y. Hakamata, T. Yoshizawa and T. Kodaira, IEICE Trans. Commun., Vol. E77-B, No. 10, October 1994.
This book describes a 2×2 switch that has an input/output unit for converting light from a single mode optical fiber to parallel rays with a collimator and has a TN liquid crystal held between two prisms, each formed by integrating a polarizing splitter and a total reflection mirror, and designed to satisfy the 1st minimum condition in the wavelength of 1.3 μm band.
However, as shown in the book described above, the wavelength used for optical fiber communication is in the near-infrared region. Unlike a wavelength used in the visible region used for a display (for example, 550 nm for green), a wavelength in the 1300 nm band or the 1550 nm band is usually used for optical fiber communication.
Therefore, a long wavelength used in the cell results in the cell thickness d being increased. This produces a problem that a conventional TN liquid crystal cell, if used for a polarization control optical switch for optical fiber communication, slows the response time. For example, the specifications for a synchronous optical network (SONET) and a synchronous digital hierarchy (SDH) define that the time to recover from a network failure be within 50 milliseconds.
Considering the future evolution of optical network systems, compatibility with conventional networks is important and so the optical switch response time must be at least 50 milliseconds or shorter. For those polarization control optical modulators conforming to the specifications, the conventional TN liquid crystal cannot be used for a liquid crystal switch without change. The details will be described below.
First, consider that the polarizing rotator of an optical switch is implemented using the TN liquid crystal technology. For the center wavelength of 1550 nm usually used for optical fiber communication, the liquid crystal cell thickness d satisfying the 1st minimum condition is about 15 μm when the liquid crystal ZLI-4792 (trademark of MERCK JAPAN Ltd.) and when Δn (refractive index anisotropy of liquid crystal) is 0.09. At this time, the turn-on response time τr is in inverse proportion to the square of the magnitude of the electric field applied to the liquid crystal cell. Therefore, the response time of 50 milliseconds or shorter may be attained by increasing the voltage even if the cell is thick. However, the turn-off response time τd is 150 milliseconds or longer at a temperature near the room temperature because the response time is in direct proportion to the square of the liquid crystal cell thickness. Thus, a liquid crystal cell having the conventional configuration cannot be practically used directly as an optical switch used for optical communication.
Consider that a polarizing rotator is implemented by an optical switch in which a liquid crystal cell is used as the 0th-degree half-wave plate. In this case, because an anti-parallel orientation or parallel orientation liquid crystal cell is used as the half-wave plate, the cell thickness is as follows:d=λ/(2·Δn)Here, if Δn of the ZLI-4792 is 0.09 and the center wavelength λ is 1550 nm, the cell thickness d is about 8.6 μm. In this case, when the response time is compared with that obtained when a TN liquid crystal cell is used, the turn-off response time τd is about 60 milliseconds, that is, about three times as higher. However, the problem is that the wavelength bandwidth in which the cross-talk attenuation increases becomes narrowed.